In most engines for a power saw, a power cutter, a lawn mover and similar consumer products, the A/F ratio is manually controllable when the engine is idling, e.g. the electronic control system is only active when the engine is at working speed or above. It would therefore be desirable to have a simple, non-expensive but efficient electronic control method, without the need of adjusting the fuel or air supply manually, when the engine is idling.
EP 0 715 686 B1 describes a method of controlling the engine A/F-ratio. Initially, the A/F-ratio is changed briefly. This could be effected for instance by briefly throttling or stopping the fuel metering. In connection with the change, a number of engine revolution times are measured. The revolution times relate to engine rotational speeds chosen in such a manner that at least one revolution of the engine is unaffected by the change, preferably an engine rotational speed that is sufficiently early for the A/F-ratio change not having had time to affect the engine rotational speed. Further at least one forthcoming revolution of the engine is chosen in such a manner that it is affected by the brief A/F-ratio change. In this manner it becomes possible to compute a revolution-time difference caused by an A/F-ratio change. On the basis of this revolution-time difference a change, if needed, of the mixture ratio in the desired direction towards a leaner or richer mixture is made. Thus using this method an optimal mixture can be achieved by testing how the engine reacts to a leaner or richer mixture. However this control is somewhat slow and mainly suitable for controlling the engine at working speeds.
PCT/SE06/000561 describes an idle speed control where the engine is started with a rich fuel setting and where the fuel setting is gradually moved towards a leaner setting until an engine speed interval is reached and if the engine speed comes above the engine speed interval the fuel setting is gradually moved towards a richer setting. It also describes a method for idle speed control using a single engine speed value where the fuel metering is decreased when the engine speed is below the engine speed value and increased when the engine speed is larger than the engine speed value. This method will find a desired engine speed; however the A/F ratio may come far from an optimal A/F ratio.
U.S. Pat. No. 6,769,394 describes a method for controlling the fuel supply to an internal combustion engine. An interval is allocated around a desired parameter value, e.g. the engine speed. When the measured parameter crosses the lower and/or upper threshold from below to above the fuel supply is cut off. And when the measured parameter crosses the upper and/or lower threshold from above to below fuel supply is switched on. The method can be used at idle. This method will fluctuate around a desired engine speed; however the A/F ratio may come far from the optimal A/F ratio.
EP 0 799 377 describes a method characterized primarily in that in the fuel supply system a fuel shut-off is effected during a part of the operating cycle by means of an on/off valve shutting off the entire fuel flow or a part flow, and in that the shut-off is arranged to take place to an essential extent during a part of the operating cycle when the intake passage is closed and consequently the feed of fuel is reduced or has ceased. This means that the amount of fuel supplied can be precision-adjusted by a slight displacement of one or both of the flanks of the on/off valve shut-off curve; this method will be referred to as Pulse Width Modulation (PWM) of the fuel supply. However, EP 0 799 377 also suggest that in particular for crank case scavenged two/four-stroke engines, the shut-offs can be performed every other, every third or possibly every forth engine revolution instead upon each engine revolution, in the case of a four-stroke engine, half as often. Of course the on/off valve could also be set to be open every revolution. In that case a major fuel amount adjustment is made instead, for instance by completely shutting off the fuel supply for a revolution. This can be done since the crank case in crank case scavenged two-stroke engines or crank case scavenged four-stroke engines can hold a considerable amount of fuel and consequently serve as a levelling reservoir, it is therefore not necessary to adjust the fuel supply for each revolution when controlling the fuel supply to the engine, i.e. adjusting the fuel supply in one revolution will affect the subsequent revolutions.